Storage-type color display tube



E O. LAWRENCE HAI. 2,943,230

STORAGE-TYPE COLOR DISPLAY TUBE June 28, 1960 2 Sheets-Sheet 1 FiledMarch 11, 1958 INV ENT ORS 7A/57' 0. ANPE/VCE @4V H. LEE

BY I

June 28, 1960 E 0- LAWRENCE ETAI- 2,943,230

' soRAGE-TYPE COLORA DISPLAY TUBE Filed March 1l, 1958 2 Sheets-Sheet 2irren/ir.;

2,943,230 STORAGE-TYPE coLoR DISPLAY TUBE Ernest O. Lawrence, Berkeley,`Calif., and Ray H. Lee,

New York, N.Y., assignors to Chromatic Television Laboratories, Inc.,New York, N.Y., a -corporation of California Filed Mar. 11, 195s, ser.No. 720,714

@Claims-Lg (cl. 315-12) This invention relates to cathode-ray tubesofthe storage type, adapted for the display of images in color fortelevision, radar, and like purposes.

A storage-type cathode-ray display tube, as the term is used herein,meansacathode-ray `tube wherein the image tracedY by a scanningcathode-ray beam, instead of`being displayedinstantaneously, a point ata time, is Veffectively stored or retained for a greater or less periodso that thepath traced bythe beam can be reproduced and displayed afterthe beam has passed. 'I'he period for which the image is retained canvary between wide limits; say, from go second, corresponding to theperiod of field repetitionof television signals transmitted inaccordance with present United States standards, or'itA can be l formuchlongerperiods, such as minutes, hours, or even days. Display of the`image can be substantially co'ncurrent with its production or jwriting,the display of any individual areaA1 of' the image iield beginning atythe instant `that it is stored and continuing until that elementary areais again scanned by the` electron beam, or the image may be held,invisibly, or remembered until it is reproduced at a later time.

Numerous storage-type monochrome display tubesy have been suggested inthe past; many of theseY arev described in Storage Tubes by KnollandgKazan (lohn Wiley and Sons, Inc., 1932), together withtheir generalprinciples of operation and including the theoretical discussion ofvarious ways of writing, reading, and erasing the images produced andreference is made to this workfor matters not discussed in detailherein. Obviously tubes of the storage type are considerably morecomplex than the simple type of cathode-ray display tubes as used intelevision receivers generally. Numerous types of cathoderay tubes forthe display of television or like images in color have also beenproposed. These, too, have involved greater complexities than monochromedisplay tubes. Furthermore, in both storage tubes and color tubes thepositioning of their various elements and the relative potentials atwhich these elements are operated are usually quite critical. Certain ofthese requirements of the two types of tubes have proved to be diiiicultto combine in a single devicel that would unite the purposes andadvantages of both.

The broad purpose of the present invention is to pro` vide a cathode-raytube eiectively combining the characteristics of storage and color typedisplay tubes. More specifically, among the objects of the inventionv(not necessarily listed in the order of their importance) are thefollowing:

(1) To provide a storage-type tube which will display a polychrome imagewith a definition substantially equal to that attainable by directdisplay;l

(2) To provide a cathode-ray tube that will` display images in color atgreatly increased brilliancy as compared with that attainablewithrdirect display;

('3) To provide a storage-type tube for displaying images in color whichis capable of either displaying Patented .lune 28, -1960 Y such imagesconcurrently with their tracing or of retaining such images for a moreor less indefinite period and later displaying them in color withsubstantially n'o degradation in resolution;

(4) To provide a tube adapted, without structural change, tousesubstantially any of the known techniques of writing, reading anderasure of the images; depending upon the particular useto which thetube is to be put; and 5 To'provide a tube which can readily beconstructed in either two color or three color versions. w In order todescribe-the present invention in general terms it is simplest toconsider rstthe known type of color television tube as proposed by oneof the present inventors and described in his Patent No. 2,692,532, Sucha tube comprises the usual evacuatedV envelope,

, preferably of generally funnel-like shape, having a view-,-

`beam bidirectionally toV scan the viewing area may be ing area orWindow inthe largeend' of the funnel and an electron gun in the neck ofthe funnel and adapted to direct aj narrow, collimated beam of electronstoward the center of the viewing area. Means for deiiecting theincorporated within the tubev in the form of deflecting plates, but aremore usumly provided by auxiliary coils encircling the neck tothetubewhen in use; the same isv true of-` means for focusing the beam intoits narrow I pencil-like form. The display screen in such a` tube iscomprised of narrow strips of pliosphors emissive of light of differentcolors upon electron impact, these strips extending: across the full'widthof the viewing area in one dimension and each strip beingl lessthan one elemental area offth'e picture t`o be reproduced in width.`Deposited over the `layer`-`of'phos`phor strips is a thin film'ofconduct-y ing material; preferably of aluminum or some other metalhaving a lowl atomic number, this film being sothin 'as to be`electron-permeable.`

Mounted between the electron gun and the screen, closely adjacent Itoand parallel tothe latter, isa structure adapted to set up electricfields'that will form a n1ulti`v plicity of cylindrical electron lenses;i.e., electron lenses which tend to converge inf` one dimension,electrons entering' 'the pupils thereof, leaving the 4component of thedirection of their paths in the other Vdimension either unchanged or,sometimes, slightly divergent. This electron lens structure may takeseveral forms,` but it necessarily includes a color-control gridcomprising two mutually insulated, interleaved sets of tightly stretchedlinear conductors, the conductors in each set being electricallyconnected. These conductors are so arranged that those of one set areelectron-optically centered over phosphor strips emissive of one colorupon the display screen while those of the second set are similarlycentered over or alined with the centers of the strips emissive of adilerent color. By electron optically centered is meant that when normaloperating potentials are appliedvto the'variplied to the two sets ofconductors of the `color grid, thus deflecting the focal point towardoneor the other of the strips o f phosphor above'which the gridvconductors are.A

centered.

InY accordance with the present inventionthe electrn lens system foraccomplishing color control and focusing comprise rst, a colorcontrolgrid as above described;

second, ascreen grid, which may be either a'rriesh of' Y ne wires or,preferably, a series of parallel wires extending transversely to thoseof the color control grid. Beyond these two grids, positioned in theplane that would normally be occupied by the display screen in a tube ofthe non-storage type, is a storage mesh comprising a foraminated sheetor netting of conductive material, coated, at least on the side facingthe electron gun, with an insulating lm having good secondaryelectronemissive properties. The display screen itself is of the sametype and conformation as would be used were it positioned in the planeof the storage mesh; it is spaced slightly beyond the storage mesh in aposition as though moved back without rotation from the plane of thelatter in a direction normal to its own plane.

Mounted between the screen ygrid and the storage mesh are iirst, asource of flood electrons, so directed as to form a space charge betweenthe ,screen grid and the storage mesh, and second, closely adjacent tothe storage mesh, a collector grid of ne wires. The preferred forms ofthe various elements described and the mode of the operation of thedevice will be explained in connection with a detailed description thatfollows, this description being illustrated by the accompanying drawingswherein:

Fig. 1 is a diagrammatic illustration of one embodiment of theinvention, not to scale, the dimensions of the various elements beingdistorted in order to show more clearly the construction and dispositionof the parts;

Fig. 2 is a diagram illustrative of the voltage gradients within thetube of Fig. 1; Y

Fig. 3 is a schematic diagram showing the connections of one form offlood electron source;

Fig.` 4 is an'elevation, partly in section, showinganother type of iloodelectron source; and

Fig. 5 is a cross-sectional view of the source illustrated in Fig. 4,the plane of section being indicated by the lines 5-5 of the precedingligure. l

The form of the present invention illustrated in Fig. l comprises theusual, generally funnel-shaped envelope 1, which may -be either of metalor glass; if of glass, it is provided with an inner conductive layer.The larger end of the envelope is closed by a glass window 3; the smallend terminates in a tubular neck 5, also usually of glass, within whichis mounted an electron gun. The latter comprises a cathode 7, usually anintensity controlling grid 9, and one or more anodes 11, the last ofwhich connects to the envelope or its conductive lining. Many forms ofthe electron gun are known, any of which may be used provided it isadapted to form a collimated narrow beam of electrons -directed normallytoward the viewing area. Connections from the various elements in theelectron gun are made to pins 12, carried by a base on the neck of thetube. In use, the usual focusing and deflecting coils surround the neckbetween the end of the anode 11, and the start of the are of the funnel.

The remaining elements within the envelope are all substantially planarin form and nearly equal in area and are mounted very closely adjacentto the viewing window 3, in planes substantially normal to theundeilected path of the beam from the electron gun to the center of thewindow. The irstof these elements is a color-control grid, comprisingtwo mutually insulated interleaved sets of tautly-stretched, fine linearconductors 131 and 132, the conductors of each set being electricallyconnected together as shown schematically in the diagram. wires of about4 mils diameter, spaced 10 to 2O mils on centers, depending on the totalsize of the tube and the area of its display screen.

The second element in the'electron lens system isa screen grid 15,formed of linear conductors similar to those forming the color-controlgrid but extending transversely to the latter across the tube.

they need not be quite as closely spaced as the control In a typicaltube these conductors will be l All of the screenV grid conductors areconnected together, however, and

grid conductors. If desired, a wire mesh may be used for the screen gridinstead of the transverse conductors only, but the latter arrangement ispreferred. The spacing between the color-control grid and the screengrid Will typically be approximately about 1/2 inch.

Behind the screen grid again is a flood cathode 17. In Fig. 1 thiscathode is indicated symbolically as a resistor; actually it is formedas a grid or network of thermionically emssive wire, e.g., ne thoriatedtungsten, connected as shown' in Fig. 3. In Fig. 3 the emissive wiresare those running laterally of the ligure and indicated at 171. Theseare cross-connected to form a network through equalizers K172, the wholebeing supplied by leads 19. As will be seen, the structure and method ofconnection is such as to put many short laments in parallel connectionso as to minimize the voltage diierence between the various parts of thenetwork and make its average potential as nearly uniform as possible,the total variation in voltage over the entire area of the cathode beingonly the order of 2 or 3 volts at most. Further, to minimize suchvoltage variations, the equalizers are made of low resistance conductor.The heating current for the elements is provided by a pair of batteries21 and the bias source that establishes a mean operating potential ofthe cathode, in relation to the other elements of the tube, connects ata point'23 between the two batteries. In a typical tube, from which thedimensions of that herein describedrare taken, the spacing of thecathode from the screen grid is approximately 0.2 inch.

The next element of the tube, spaced, in the present case, 0.2 inch fromthe flood cathode 17, is a collector grid 25. This should be a mesh ofvery ne wire, 3 mils in diameter or less, the openings betweenv thewires being relatively large in comparison With'the wire diameter; e.g.,0.10 inch. Alternatively, it may be stretched parallel wires runningunidirectionally, without the transverse members of the mesh.

A storage mesh 27, upon which the operation of the tube primarilydepends, is spaced 0.1 inch from the collector grid in a plane parallelthereto. This mesh is very tine. It may be woven wire, 250 or moremeshes to the inch, or it may be a thin foraminated foil, formed, forexample, by the etching process, with its foraminations spaced oncenters by distances ot the same order of magnitude as the Woven meshjust mentioned. The storage element itself is deposited on theconductive base formed by the mesh or foil. It comprises an insulatinglayer which may, for example, be deposited on the conductive oase byevaporation. Any of the numerous materials that have been suggested forthe purpose may be used; these materials have in common that they have arelatively high secondary emission ratio and high insulation resistance.Among those that have been suggested and are, in fact, suitable, are thephosphors, such as the silicates, evaporated silica and aluminum oxide.This coating need only be deposited on the side of the mesh facing theelectron gun.

The nal element of the tube is a display screen, generally indicated by:reference character 29, mounted just within the viewing window 3 andapproximately 0.25 inch from the storage mesh. It comprises a glass orother transparent baseplate 31, on which, facing the electron gun, isdeposited a layer of phosphors 33 covered by a conducting film 35. Thephosphor layer comprises strips of phosphors emissive of differentcolors upon electron impact. These strips run substantially parallel tothe wires of the color grids 131, 1&3. The number of different phosphorsemployed may be either two or three, preferably three if the device isto be used to reproduce colorrtelevision signals, although in certaincases two color reproduction may be used for certain military, scientic,or other purposes. For a bi-color tube s trips of one color, say red,are electron-optically centered behind conductors 131 of the color grid,while those: of a. secondrcolor, say; green, are similarly centeredbehind: conductors 132. For a tri-color tube strips of` the third.color, say blue, are positioned midway between each pair of red andgreen strips. The order `of the strips is, underthe above assumptions,red, blue, green, blue, red, etc., there being twice the number of bluestrips as of red or green. Which strip is chosen. as the center strip is`to a large degree arbitrary and d'epends on the particular system withwhich the tube is used. The order described is purely illustrative.

The strips are electron-optically centered beneathV the color controlgrid conductors, not physically centered; except` at the 'center of thescreen a perpendicular dropped from the conductor 131 would not strikethe center of. the corresponding strip. What electron optically centeredmeans, in thepresent instance, must Ybe derivedindirectly from theposition. such .strips would occupy were the display screen moved to theposition of `the storage-meshfzlg; As will `be described in more detail.later, the electrons streamingv from the `gun emerge from it at avelocity corresponding to approximately 3 kilovolts, and areacceleratedto 5 kv. at' the screen grid, beyond. which-their velocitiesare substantially constant until theyy strike'the storage mesh. Theacceleration does not aiect the vpaths of the electrons traveling downthe tube axis. iWhen` deflected toward the edges of the Screen in thescanning process, however, the accelerating forces. are predominantlynormal tothe surfaces of the grids, the transverse components ofthevelocities being but littleaffected. .Their trajectories can becomputed; as has. been des'cribedin the prior art, andthe pointi atwhich? an electron `of the beam which would pass through the positionvoccupied by one ofthe grid con-- ductors.;131.. (were it; not there)would strike the storage mesh is therefore electron-optically centeredor alined behind that conductor.

The electrons used to excite the screen, however, are not those of thebeam but thoseA emitted trom the floodz cathodes 17 They are Ynotsubject to dellection buttravel ini-paths perpendicular to the surfacesof the storage mesh` and screen. The disposition of the strips` on theAscreen is therefore what it would be were the screen moved axially ofthe tube without rotation'` about its axisinto the-position lof thestorage mesh.

Separate connections are brought out of the tube for each of the sets ofcolor-grid conductors, the screen grid, the flood cathode (as hasalready been described), the collector grid, the base conductor of thestorage meshand the conducting lm 35 of the displayscireen. Some ofthese connections can be brought out through the pins 12 on the base; inother instancesit is more convenient to bring them out through thewallsof the tube. These connections are shown schematically, withoutnecessarily indicating how they would be handled in any individual tube.

There are various ways of writing the signal on the` storage mesh, alldescribed in Storage Tubes by Knoll and Kazan, cited above. For purposesof the present'description the. operating parameters rst described willbe those suitable to non-equilibrium writing and grid control readingVasdescribed therein, it being funderstood that grid control reading willbe utilized irrespective of the writing method actually employed. Thereare two setsvof conditions that must be met which dictate the biasesimposedupon the various electrode structures; rst,the electron lenssystem established by the colorcontrol grid l13 and the screen grid V15must have a focal length of the proper magnitudefto converge electronspassing between adjacent wires 131, 132 into a line focus,

or approximate focus, giving a beam width lnot greater thanone-.half'lthe width "ofone of the phosphorl strips. and preferablyless. The focal length of the lens'system' here described depends uponthe ratio between the initial acceleratingvoltage applied-tothe beam andthe voltage betweenthecolor'control,grid and the screen, grid. v It is'Ybut slightly affected byV the presence of Hood cathode"` 1'7- andcollector grid 25. However, the voltage` gradients within the spacebetween the screen grid and storage mesh must be such as to permitproper writing or collection of stored charges without interfering withthe reading of the signals as they appear on the display screen.

Thevoltage gradients used to accomplish these results are indicated bythe plot of Fig. 2. It is convenient, for various reasons, to operatethe conductive base of "the storage mesh at Vground potential. VThe ploto f` voltage gradients is based' on the assumption that it is sooperated but it is to be understood that under other circumstances anypoint of the system c an be. grounded, as long as the voltagedifferences between thevarious elements are in the proper ratio. i i Inthe operation'here illustrated the cathode7 is biasedy at about 5 kv.negative toground andY the control grid -9 about 30 volts negative tothe, cathode. The final anode 11 of the electron gun is biasedapproximately Skv. negative to ground; in practice betweenabout 2.7 and3.3, it being this voltage that is varied to obtain the best focus ofthe beam by the color grid, both sets offconductorsv of the latter beingoperated at the same mean potential. TheV screen grid is grounded, sothat there is approximately a 3 kv. dierence in potential between it andthe average potential of the color control Vgrid conductors. In use, thecolor switching voltage may be appliedto the .color-control gridconductors through a transformer 37, the secondary-whereofis provided"with a center tapthrough which the bias potential is supplied.

With the voltage gradientsjand spacing of .the parts as here described,the switching can be accomplished with an applied .A.C. voltage acrossthe color grid conductors of approximately l00"volts.V

The flood cathode 1'7 is operated at a bias potential at about. l0'voltspositive and the collector grid Vabout 150 volts positive -to ground.The base potential of'the sto.- age meshe'is, as before stated, zero.Except for the charges stored on` the individual elements of the storagemesh, there is/thereforeV no` net potential difference between thescreen grid 15 and the storage mesh 27. The maximum voltage differencewithin this space Vis there- Vfore only in the neighborhood of volts."This is so Small in comparison with the 5000 electron-volt energy oftheincoming electrons that it may be disregarded and the beam considered asthough it were traveling in a eld-free space; i.e., the eiectV of theood cath'ode and the collector grid on the path of the high velocitybeam is so small that it may bev neglected.

With the materials used fo'r coating the storage grid, the 5 kv. energyapplied .to the electron beam is substantially that required to operateit at the so-called second cross-over point of secondary electronicemission of the coating material (Vm, to use the notation generallyemployed); i.e., at the impact energy whereat the secondary electrongun. `Under the circumstances here considered these charges are alwayspositive.` In scanning, therefore, the beam deposits, on the storagemesh,A acharge pattern that corresponds inintensity to the' lightpatternV thatwould be exhibited vwerethe display screen moved' to thesame position. The film 35VA on the surface of'the display screen. is

biased to a relatively high potential positive toV ground;` somewhere inthe order ofV 10 kv., but this is not critical,

The flood cathode 17 liberates electrons which, since the ood cathode ispositive to both the screen grid and the base of the storage mesh, butnegative to the collector grid, are attracted toward the latter, most ofthem passing through the meshes to form a substantially uniformlydistributed space charge immediately adjacent to the secondary-emittingsurface of the storage mesh and thus forming a virtual cathode betweenthe storage mesh and the collector grid. This virtual cathode, theindividual charges on the storage mesh,'and the conductive film on thedisplay screen therefore form, in effect, a triode, with the storedcharges acting as control signals on the grid to control the electronflow.

The graph of Fig. 2 illustrates, only very roughly to scale, potentialgradients within the tube, the dotted lines indicating the approximatevpositions along the Vaxis of the tube whereat the various bias voltagesare applied. The difference between the cathode voltage E and the biasEi at the intensity-control grid is too small to show on the scaleselected.` The potential of the anode 11 is substantially equal to thebias voltage El applied to the color control grid, so that through themajor portion of the tube the beam travels thro'ugh a substantiallyfieldfree space. A steep potential gradient accelerates the beam betweenEl, the color-control grid potential and E2, the screen-grid potentialat ground. The storage mesh is also biased to the same potential E'2,but between these elements a very small potential gradient existsbetween the screen grid and the collector grid biased at EC. Thisgradient is shown on a somewhat exaggerated scale, but even at thisscale, the volt rise from E2 to' E, is too small to show. Beyond thestorage mesh is the steep gradient rising to l0 kv. at E3.

Because of the extended parallel surfaces of the storage i mesh and thedisplay screen, the lines of force between the two are normal to bothand the paths of electrons penetrating the mesh of the storage screenfollow the lines of force, so that the image displayed upon the displayscreen is substantially identical with that which would be shown upondirect scanning. It should be mentioned that the display screen may beformed directly on the window 3. The window is almost always slightlycurved to enable it better to withstand the air pressure on its surface.If the curvature is slight it will have little efect on the electrontrajectories between the storage mesh and the screen, as the lines offorce are nearly normal to the -storage mesh until the electrons havereached nearly their terminal velocity. If the screen is more deeplycurved the trajectories can be computed and allowance made inpositioning the phosphor strips.

The pattern displayed may, however, be very much brighter than that of adirectly scaned display screen, depending upon the way in which the tubeis used. Any of the various known methods of erasing an image that hasbeen written upon the storage mesh in one scanning, preparatory to' there-storage of a new image, may be used. These have been described in thetext above referred to.

One method, involving a different mode of writing, is of particularinterest; i.e., equilibrium writing.

In this method of writing the cathode 7 is operated at a potentialnegative to the storage mesh somewhat greater than Vm, and anunmodulated beam of high intensity is employed, the grid 9 being coupledback to' the cathode 7 through a condenser in order to `insure that nomaterial A C. potentials appear between them to cause variation of beamintensity, while still permitting the proper dilference in biaspotential between the two. The collector grid is operated at about thesame or at a slightly lower potentialdiference with respect to thegrounded base of the storage mesh as in the case previously described.

Under these circumstances the elementary areas of` the storage mesh thatare under bombardment from the scanning beam will tend to assume anequilibrium potential such that the secondary electrons to the collectorgrid are equal in number to the primary electrons from the beam. Thisequilibrium potential is a constant for a given structure and cathodepotential. The quantity of the charge required to bring any element ofthe storage surface to equilibrium depends upon the potential differencebetween the collector grid and the conducting base of the storage mesh,still assuming constant cathode potential; it will vary with the latter.

The signal voltage can therefore be applied either to the cathode, thecollector grid, or the storage mesh, therebeing some slight advantage inapplying it to the cathode as this does not aiect the triode operationof the ood cathode-storage-mesh-display screen combination. Any one ofthe three will work, however, provided the signal is applied in theproper polarity.

lf cathode modulation is used a positive signal will decrease the impactvelocity of the beam, and since operation is beyond the cross-overvoltage Vm, will increase the secondary emission, and thus tend to drivethe impacted area positive, increasing the brightness of thecorresponding area of the screen.

Applying the signal to the collector grid, the charged surface 4tends toassume the same potential as the grid and in this case also a positivesignal corresponds td a positive charge and increased brightness of thecorresponding point on the screen.

Applying the signal to the storage mesh the reverse is the case; with apositive signal the charge collected on the storage surface must benegative to restore equilibrium, resulting in fewer electrons passingthe mesh in the triode operation of the device. The signal polarity musttherefore be reversed as compared with a gridmodulated direct displaytube or the other two methods of operation.

In this last-described mode of operation, the oodelectron flow will varyconstantly with the variation in storage-mesh voltage. Over the periodof `a frame, however, the electron ow will correspond to the averagevoltage of the mesh, varying above and below the zero axis of the A.C.component of the signal. This method of applying the signal is thereforeoperative, although less elegant than the others.

With all three modes of equilibrium writing the storage-mesh elementsare recharged (or discharged) to their new equilibrium potentials eachtime they are scanned by the beam, thus erasing their former charges.The image on the screen is therefore displayed without interruption,giving a picture without icker or color crawl.

In equilibrium writing one of the principle concerns is to obtain asuiciently intense beam to obtain complete equilibrium. High currentbeams tend to diverge, because of the Coulomb effect. ln the presentdevice the electrons passing between any pair of color grid wires arere-focused to an area much smaller than the initial cross-section of thebeam. Therefore higher densities and better approach to equilibrium canbe attained than with most devices using equilibrium writing.

Another method of introducing the ood electrons into the space betweenthe screen grid and the collector grid is illustrated in Figs. 4 and 5.In this arrangement the source of the flood electrons surrounds, atleast partially, the area into which the electrons are introduced. Theelectron source is a gun comprising a glass frame 41 of U- orchannel-shaped cross-section, with the open end of the U directedinward; this member 0r frame may either be mounted within the envelopeor sealed in its walls substantially in the plane of the cathode 17 ofFig. 1. Within the U section of the frame, starting at the bottom of thechannel, is a heater element 43, an

electron emissive cathode 45 and an anode 47, slotted to permit egressof the electrons from the cathode.

In operation the anode 47 Vis biased a few volts posanni; reg-heeft 9.itive to ground potential, the` cathode'Y slightly :less positive, theactual values of the biases being'ldetetminedkby expeimentfat the bestvaluesto'give annifonr distributiontotheinjected-'electrons'. Y.

" arangementhas considerable exibility Ainloperationf. Thev cathode canbe sectionalized v.so that the various sections can bei-operated' atdifferent `'biasesjto secure the `best and most` iunifnrrn` distributionof- =elec mms :within the region between the 'sc/reenggridfandfthe grid.in someiu'stances `it possible to omit entirely the cathode'sections'othe' shorter sidesy of the vrectang'ular'frame, provided Varectangular `tube is used. lWith the cathode operated at apotentialslightly positive to the `bias potential -of the storagemesh,rand laccelerated t'owardftheilatte'r -by the collectorl grid,electrons will approach-the storage-'meshfasfa result of theiracceleration but not strike yit as long'as n'o' part` offitsxsurfacereaches `a .potential thatfisasufar positive as that of-the cathode;4 Asa result,-if thesupply `f electrons Vfrom the flood cathode isadequate-fthe desired virtual cathode will form immediately infront of'thel storage fmeshbut will penetrate the latter only to the extent thatthe iield from the display screen 4penetrates it, :the effectivepenetration, ofcourse, being moditedby, the charges collected by thestorage plate. 4

"It will be appreciated that the electron flow through thestorage mesh`need not be large in Aorder to produce fdisplay that is very brilliantin comparison with 'the one produced. by a scanning electron beam.In'they ordinary television display tube the current carried by theelectron beam is only a fraction of a milliampere. At the rate ofscanning dictated by present U.S. standards of transmission `the beamfalls upon any individual area for only about Immo@ of the total timeduring which the picture is displayed. For intervals of the order ofmagnitude of the frame frequency the eye integrates the light itreceives from a given area. 'Ihe light emitted from the display screenis, to at least a first approximation, proportional to the energy inputto the screen. It will thus be seen that even if the display screen isoperated at a voltage of only one-half that utilized between the cathodeand screen of a tube using direct scanning, individual areas of thestorage mesh need pass only about 174200,00() of the current in thescanning beam to produce equal brightness. The actual current densitymay be a great deal higher than this, giving a high degree ofbrilliancy. Some electrons of the scanning beam will, of course,penetrate the storage mesh and strike the screen but these are so smallin number in comparison with the electrons from the iiood gun or cathodethat 'their effect `on the display is negligible.

If the equilibrium method of writing and erasure are employed,television images can be produced without ilickers; because the mesh is'always negative to the flood cathode it collects no electrons from thelatter and the image fades or deteriorates only as a result of theleakage through the insulating, secondary-emissive material. Since theeffective resistance of the material is very high and the voltagedifferential between the charges collected upon it and its biasingpotential are small, the rate of leakage can be very low and the storedimage maintained for long periods. This is advantageous in manyapplications of the device where a record is to be produced for laterviewing. It is also possible to use alternate writing and reading anderasure in the same manner as has been proposed for monochrome storagedisplay tubes. Since the techniques for this type of operation arewell-known it appears unnecessary to describe them in detail here. p

While specific voltages, electrode spacings, grid wire i dimensions andphosphor stripwidths have been given in this description, this has beendone for illustrative purposes only. Diierent secondary-emissivematerials have different cross-over points and since -in most methodsItif-operation the-potential diderence between the cathode of theelectron gun and-.the conductive base.` of thestorage-niesh shouldbesubstantiall'y equal to the cross-over `point voltage, .differentvoltages' maybe required for different secondary emissive materials. Inthis casethe' bias voltages applied to other elements within the `tubewill have to be changed. The focal length. of an' electron lens dependsupon the ratio of Ithe voltages applied .to its various elements and notupon their a-bsolute values. Thus, if the second cross-over voltage of-aparticular material usedl for the insulating coating of Vthezstora-gemeshwere found to be, say, 3000 vol-ts instead-'of 5000; the only:change required to produce the results here, described `would be ascaling down of the vol-tages, Boland E1 to 3/ 5 ofthe valuesgiven; Le.,to 3 vkvlvffor Bland 1.8 kv. instead of 3 as, the normal valueof,E1`.cff."f.sV .fr

'Thefspacings between. the color grid, screen grid, and storage meshmay. be changedif 4sui-table changes are made` in the relativepotentials appliedV to them. A steeper,potentialgradient between colorgrid and screen grid will shorten the effective focal length of theelectron lenses and per-mit closer spacings. 'Lower velocities betweenthe electron gun and the color grid will permit lowerV switchingvoltages. All of these relationships are now Well-known and electronlens systems can be devised in accordance with known principles to meetspecific ments (a) to (g), mounted in succession between said V electronIgun and said viewing area and defined as follows:

Element-t (a): a color-control grid of substantially equal dimensions tosaid viewing area comprising two interleaved and mutually insulated setsof parallel linear conductors, the conductors of each set beinginterconnected and the spacing between adjacent conductors of .therespective sets .bein-g of the order of magnitude of an elementary areaof the image to be displayed;

Element (b): a screen grid mounted parallel and adjacent to element (a)and comprising parallel conductors eX- tending in a direction.transverse to the conductors of element (a);

Element (c): a source of dood electrons a space charge element (a);

Element (d): a collector grid mounted in a plane parallel to elements(a) and (b) and of substantially like dimensions thereto;

Element (e): a charge-storage mesh mounted parallel to element (d) iandcomprising a foraminated base .of conductive material having a coatingof insulating secondary-emissive material adherent thereto on at leastthe side thereof vfacing element (d);

Element (f): a display screen mounted parallel .to element (e) andcomprising a light transmissive base, a layer of phosphors thereoncomprising parallel strips of phosphors emissive on electron impact oflight of different colors, strips of one color being alined electronoptically behind the conductors of one of the sets `of element (a) andstrips emissive of a different color being so aligned behind theconductors of the other set thereof, and `an electron-permeableconductive layer overlying said phosphor layer; and i l for establishingbehind `element (b) as viewed from Element (g): leads for applyingdifferent electrical poy element (a), to elements (b), (c), (d), (e),and the conductive layer of element (f).

2. The combination as defined in claim 1 including,- in addition, meansfor varying Ithe intensity of said electron beam.

3. In a storage-type cathode-ray tube for the display of images incolor, t-he combination, within an evacuated envelope including aviewing area at one end thereof and an electron gun at the opposite endthereof adapted to develop a concentrated beam of cathode rays directedtoward said viewing area, of elements designated as elements (a) to (g),mounted in succession between said electron gun and said viewing areaand dened as follows:

Element (a): a color-control ygrid of substantially equal v dimensionstosad viewing area comprising two inter- Y leaved .and mutuallyinsulated sets of parallel linear conductors, Ithe conductors of eachset being interconnected and lthe spacing between adjacent conductors ofIthe respective sets being of the order of magnitude of anelementaryarea of the image to be displayed;

Element (b) a screen grid mounted parallel and adjacent to element (a)and comprising parallel conductors eX- tending in a direction transverseto the conductors of element (a) Element (c): a cathode comprising agrid of thermoemissive iilaments, said cathodehaving over-all dimensions substantially similar to elements (a) and (b) and being mounted ina plane parallel thereto;

Element (d): a collector grid mounted in a plane parallel to elements(a) and (b) and of substantially like dimensions thereto; Element (e): acharge-storage mesh mounted parallel to element (d) and comprising aforaminated base of v conductive material having a coating of insulatingsecondary-emissive material adherent thereto on at least the sidethereof facing element (d); Element (f): a display screen mountedparallel to element (e) and comprising a light transmissive base, alayer of phosphors thereon comprising parallel strips of phosphorsemissive on electron impact of light of diierent colors, strips of oneycolor being alinednelec- VItron optically behind the conductors of oneof the sets of element (a) and strips emissive of a diiferentcolor vbeing so alined behind the conductors of the other set thereof, and anelectron-permeable conductive layer overlying said phosphor layer; andElement (g): leads for applying diiferent electrical potentalsrespectively to the ytwo sets of conductors of felement (a), to elements(b), (c), (d), (e), and the conductive layer of element (f).

References Cited in the le of this patent UNITED STATES PATENTS2,721,293 Gow Oct. 18, 1955 2,748,312 Beintema May 29, 1956 2,795,727Hae June l1, 1957 2,867,686 Hafner Jan. 6, 1959

